This proposed work is in two parts. Part I involves full temperature magnetochemical studies on various derivatives of fully oxidized cytochrome oxidase for the purpose of determining the magnitude (-J value) of the antiferromagnetic exchange interaction between iron(III) and copper(II) at the binuclear [Cyt.a33+(bridge)Cu(u)2+] active site. The two deeerivatives of the enzyme proposed for our continuing studies are the formate derivative where Cyt.a33+ is S=5/2 and the bridge likely HCO2-, and a resting derivative (anaerobically prepared) where there may be no bridging ligand between Cyt.a33+ abd Cu(u)2+. Magnetochemical results for these two derivatives, along with our previous results for the resting enzyme (where Cyt.a33+ is a high-spin S=5/2 center) and a [resting CN] derivative (where Cyt.a33+ is a low-spin S=1/2 center) will lend further insight into the three leading possibilities for the active site structure: (1) a Mu-imidazolato case with [Cyt.a33+(imid)Cu(u)2+], a Mu-oxo case with [Cyt.a33+-O-Cu(u)2+ (and the bridge from O2), or a Mu-mercapto case with [Cyt.a33+-S-Cu(u)2+]. Magnetochemical studies such as those proposed are especially important and novel to the oxidase problem since they supply a general method, not normally available through other physical /spectroscopy techniques, for directly probing the nature of the bridging ligand itself, in addition to the metal centers being bridged. Part II will involve the synthesis, structural characterization (by X-ray crystallography and EXFAS) and study (by magnetochemistry, resonance Raman, epr, and Mossbauer spectroscopy) of various [TPP)FeIII is hs-(bridge)CuII] model compounds where the bridges are the oxidase possibilities: imidazolate, oxo, mercapto, CN-, and HCO2-. Through preliminary work, several of these novel bridged species (Mu-imidazolato, Mu-mercapto, and possibly Mu-oxo) are already in hand and are yielding information pertaining to the oxidase question. The [-J value/Fe3+ spin state/bridge] correlation that is emerging from these model systems will be used in interpreting the magnetochemical results of Part I, which will, in turn, more firmly establish the correct structure of the oxidase active site, and an important step will have been taken toward elucidating the catalytic mechanism by which oxidase reduces dioxyhgen to water plus cellular energy in the 4e- process: 02 + 4H+ + 4e- directional 2H2O.